Timely diagnosis of potential ailments is perhaps the most effective tool available to modern physicians in their battle against serious illnesses. If discovered early enough, many of the deadliest illnesses and diseases pose little threat to a patient with proper treatment. To discover an illness, physicians typically perform a careful examination of a particular part of the human body, either by an invasive, or a non-invasive procedure. An example of an invasive procedure is the biopsy, in which a surgeon removes a sample of human tissue with a needle or a scalpel. Invasive procedures like the biopsy have inherent drawbacks, such as pain for the patient, and the need to heal the area from which the tissue sample was removed. Thankfully, technological and medical advances over the past fifty years have created a number of non-invasive diagnostic procedures.
Non-invasive diagnostic techniques such as Magnetic Resonance Imaging (“MRI”), Computer Tomography (“CAT” or “CT”), X-rays, Positron Emission Tomography (“PET”) and Ultrasonography are widely used by physicians today. However, while non-invasive techniques are painless and do not require healing time, they may still pose certain dangers to the patient. For example, an unhealthy dose of X-ray radiation may lead to cancer. The strong magnetic fields produced by an MRI machine may also cause adverse health effects in the patient. In contrast with these devices, ultrasonography does not rely on electromagnetic waves or ionizing radiation. Ultrasound machines instead depend on mechanical vibrations to perform measurements.
Briefly, ultrasound machines include a transducer array, a beamformer, a processor, and a display. A transducer is a device that converts one type of energy to another type of energy. Ultrasound machines mostly use electroacoustic transducers, which convert electrical energy (voltage potential across the transducer) into mechanical energy (vibrations), and vice versa. The beamformer sets the phase delay and amplitude of each transducer element to enable dynamic focusing and beam steering. Where appropriate, a lens is mounted on the transducer array to focus the transmitted pulses and received echoes. In operation, the transducer array sends out a number of pulses directed toward the anatomical area of a patient to be imaged, and after a certain propagation delay receives echoes that were reflected back by the patient's anatomy. The received signal can then be presented on a display for immediate examination or recorded for a later review.
Over time, the industry has developed a commonly understood terminology for describing various components of an ultrasound machine. The various combinations of transducer arrays and multiplexers were in particular need of a common term, due to the different goals and performance attributable to each combination. While terminology used by the industry is generally agreed upon, certain variations exist, mostly regarding the multiplexing structures that connect transducer arrays to the beamformer.
The terms are generally understood by persons in the art as follows:                1D arrays have a fixed elevation aperture and are focused at a static range.        1.5D arrays have a variable elevation aperture, and either static or dynamic focusing (Industry terminology for this category differs. For example, General Electric (GE) splits these arrays into two categories: 1.25D and 1.5D. In GE terms, a 1.25D array provides for variable elevation aperture, but its focusing remains static. However, a 1.5D array, in GE terms, has a dynamically variable aperture, shading, and focusing, all which are symmetric about the elevational centerline of the array. A GE article titled “Elevation Performance of 1.25D and 1.5D Transducer Arrays” by Wildes et al., the entire contents of which are incorporated herein by reference, provides an overview of various linear transducer arrays.).        2D arrays permit focusing and steering in both azimuthal and elevational directions, with comparable results.        
Regarding actual ultrasound machines, ordinary hand-held and stationary scanners such as the ones depicted in FIGS. 1A and 1B have been used since the 1970s. As technology progressed, so did the quality of images provided by ultrasound machines. Phased arrays, such as the 1D array pictured in FIG. 2A, have drastically improved lateral and axial resolutions of ultrasound machines. Axial resolution is the minimum separation required between reflecting objects stationed in the path of the ultrasonic pulse. If two reflecting objects are too close together, the received echoes are also too close together, appearing as if they were reflected by a single object. Lateral resolution is the minimum separation required between reflecting objects in the direction perpendicular to the path of the ultrasonic pulse. While 1D phased or linear arrays improve lateral and axial resolutions, their elevation performance is controlled by using a simple lens, which leads to a more uniform slice thickness but only permits elevation focusing at a single focal distance, with a depth of focus that depends on the elevation aperture. The elevation aperture must be proportional to the focal distance, and at the same time narrow enough to provide a sufficient depth of focus. However, a narrower elevation aperture provides less effective focusing, and hence results in a lower lateral resolution.
More recent developments, such as the 1.5D array depicted in FIG. 2B, have improved elevation slice-thickness performance both in the near- and far-fields, while still using only a single beamformer for both azimuthal and elevation focusing. However, these kinds of arrays suffer from limited penetration depth, the possibility of beam-splitting caused by the shape of the lens, and also by their cumbersome and slow multiplexing structures.
Lenses with a cross-section shown in FIG. 2b, are prone to a phenomenon known as beam-splitting, because their cross-sectional depth does not take into account a wave's propagation time. For example, the lens's center row is the first to receive and quickly pass the echo through to the multiplexer. However, by the time the lens's outer rows receive and pass through their own parts of the echo, the time-frame has shifted, and it is unclear which echoes are being passed through. Thus, the beam is actually “split” into components which might not be received simultaneously by the beamformer.
Another downside of the 1.5D array depicted in FIG. 2b is its slow multiplexing structure, or more accurately its two multiplexing structures. Such an array, described in U.S. Pat. No. 5,882,309 to Chiao et al., the entire contents of which are incorporated herein by reference, actually has two multiplexers. One multiplexer controls elevation aperture growth, while the other controls azimuthal aperture growth. This results in very slow scanning, as the two multiplexers cannot be switched independently of one another.
Convex 1D arrays, such as the one depicted in FIG. 3, suffer from a very limited penetration depth and lower resolutions because their geometry requires smaller elements to sustain the same f -number at greater depths.
Turning to three-dimensional (3D) imaging, performance in elevation focusing, depth of penetration and high resolution become very important, particularly in the medical field. When using ordinary ultrasound scanners, like the one depicted in FIG. 1A, physicians and ultrasound specialists receive one or more two-dimensional images in the azimuthal plane. As mentioned earlier, modern transducer arrays capable of dynamic focusing provide a large azimuthal aperture, leading to high quality two-dimensional images. In the medical field, the same resolution quality would also be expected of 3D images. Thus, elevation focusing performance of the 2D image slices making up the 3D image becomes very important. In addition, an automated 3D ultrasound imaging machine should also provide high resolution quality at greater depths, since there is no operator to make needed adjustments, as there would be with a manual ultrasound scanner. Accordingly, there is a need to provide an ultrasound system for three-dimensional imaging, without the drawbacks associated with the prior art. To this end, it is desirable to provide a system capable of an increased penetration depth, shorter imaging time, more efficient multiplexing structure, and greater flexibility in azimuthal and elevational focusing.